1. Field of the Invention
This invention relates to a method and apparatus for combustion of liquid fuels or mixtures of liquid and gaseous fuels. In one aspect, this invention relates to emissions produced by the combustion of liquid fuels and mixtures of liquid and gaseous fuels. In one aspect, this invention relates to the control of NOx emissions resulting from the combustion of liquid fuels and mixtures of liquid and gaseous fuels. In one aspect, this invention relates to a method and apparatus for dual-fuel combustion, i.e., combustion of a mixture of liquid and gaseous fuels. In one aspect, this invention relates to dual-fuel combustion in boilers for steam and power generation.
2. Description of Related Art
To increase the cost-effectiveness of steam and power generation by the utilization of low cost liquid and gaseous fuels, including waste liquid fuels and low Btu gaseous fuels, the combustion system should be capable of dual-fuel combustion, i.e., the combustion of a mixture of liquid and gaseous fuels, while maintaining high efficiency and low emissions (NOx, CO, CO2, unburned hydrocarbons, particulate emissions, and the like) to meet the limits set by the U.S. Environmental Protection Agency (EPA) in different regions of the United States, including California. The best available combustion technology does not provide dual-fuel capability with low emissions for both liquid and gaseous fuels and, as a result, usage of liquid fuels, in particular, is limited in many areas and even prohibited in some areas, such as southern California.
Low emissions dual-fuel burners or oil burners that achieve relatively low NOx emissions, on the order of 60-70 ppmv at 3% O2, usually utilize flue gas recirculation (FGR) where a portion of the flue gases generated by fuel combustion, up to about 20%, is recirculated into the combustion chamber, thereby lowering the peak flame temperatures and the percentage of oxygen in the combustion air/flue gas mixture which, in turn, retards the formation of NOx caused by high flame temperatures (thermal NOx). Although flue gas recirculation reduces the peak flame temperature to reduce thermal NOx formation, it does not reduce the fuel-bound NOx formation. Consequently, the use of flue gas recirculation in the combustion of oil and other liquid fuels is typically limited to NOx values upward of 60 ppmv depending on the nitrogen content of the particular liquid fuel.
Conventional liquid fuel combustion typically utilizes atomization of the liquid fuel to produce liquid fuel droplets and, thus, facilitate combustion. U.S. Pat. No. 6,601,776 to Oljaca et al. teaches methods and devices for atomization of liquids for use in a variety of applications such as flame and plasma-based atomic spectroscopy, nano-powder production, particle/droplet seeding for laser-based flow diagnostics, spray drying for the production of fine powders, nebulizers for inhalation in delivery of medication, and for atomizing liquid fuel for use in combustion chambers, and teaches the use of heat-based atomization in which a pressurized liquid is raised to an elevated temperature in an atomization nozzle, resulting in a heated spray that is more resistant to re-condensation. The atomizer is in the form of a heated tube containing a pressurized liquid which is atomized at a reduced pressure, forming fine droplets as well as partial vapor. Atomization of the pressurized liquid is tailored by modifying the heating profile of the heated tube to allow controlled atomization of different liquids and/or combinations of liquids having different atomization requirements, or to adjust the mean particle size and size distribution needed for a particular application.
U.S. Patent Application Publication 2009/0005950 to Scalia, Jr. teaches a method and apparatus for uniformly controlling a combustion system by a transfer of heat to a fluidic fuel along a heat/fuel interface having a large surface area immediately prior to mixing of the fuel with air. Control of the temperature of the fuel input to an air/fuel mixing region of the combustion system is said to provide improved efficiency by an expansion of modulation ranges available for factors that together are determinative of the combustion efficiency, such as fuel flow rate, fuel droplet size, air flow, and input fuel pressure. Preheating of the liquid fuel is used to control the droplet size and distribution and rapid ignition in the air/fuel mixing region inside the combustion chamber.
U.S. Pat. No. 6,012,915 to Mori et al. teaches a method of combusting a water/fossil fuel mixed emulsion comprising elevating the temperature of the emulsion and vaporizing the emulsion, jetting the water/fossil fuel mixed gas thus formed from a burner, and bringing a Brown's gas flame of a Brown's gas combustion burner in contact with the flow of the mixed gas, thereby combusting the water/fossil fuel mixed gas. The emulsion is indicated to be a non-combustible waste having a water content of about 90%, for which the use of the Brown's gas burner is required to ignite and maintain the combustion of the emulsion.
U.S. Pat. No. 6,971,336 to Chojnacki et al. teaches a firetube boiler system comprising a pressure vessel containing two combustion sections and an in-line intermediate tubular heat transfer section between the two combustion sections. The system utilizes staged oxidant combustion for fuel-rich combustion in the first combustion section and fuel-lean combustion in the second combustion section with sufficient cooling of the combustion products from the first combustion section such that when the secondary oxidant is provided to the second combustion section, the NOx formation is less than about 5 ppmv at 3% O2. However, the substoichiometric combustion of liquid fuels using this system undesirably produces a substantial amount of soot in the first combustion chamber for which no provisions for prevention or suppression are provided.